Previous reports addressing the possible link between pesticides and breast cancer have been limited by small study sizes and restricted ranges of exposure due to a focus on urban rather than rural populations. Nonetheless, residues of DDT and other chlorinated hydrocarbons have been found to be elevated in women with breast cancer as compared to controls. The proposed study is built on the Carolina Breast Cancer Study, a case- control study of breast cancer funded by the National Cancer Institute's SPORE program. Of 900 identified cases, an estimated 800 cases will be included, divided evenly into four groups by race (black, white) and age (<50, 50+). They will be compared to 800 controls with the same race and age distribution. Nurse interviewers are collecting blood and administering a detailed interview concerning the complete array of known and suspected risk factors for breast cancer, including several items concerning pesticides. The proposed study will address pesticide exposure by extending data collection to include detailed assessment of exposure to pesticides and related chlorinated hydrocarbons. The additional exposure assessment will include a detailed interview of selected women (approximately 40% who have lived on farms) to assess historical pesticide exposures in agriculture, assays of blood samples for residues of DDT, PCBs, and related chlorinated hydrocarbon compounds, and genetic polymorphisms affecting carcinogen- metabolizing enzymes. Pesticide exposures will be compared in cases and controls to derive estimates of the relative risk associated with level of exposure to chlorinated hydrocarbons and other pesticides. Effect modification by polymorphisms in carcinogen-metabolizing enzymes will also be examined. The size, racial composition, and rural residence of the population, combined with the biochemical and interview-based assessment of exposure, represent significant improvements on past studies of this issue.